Although mainly a disease of cattle, besnoitiosis can cause infection in horses and other herbivores. This disease is caused by a tiny single-celled organism called a protozoan; in this case, the protozoan is known as Besnoitia bennetti. It has been reported in horses in Africa, southern France, and Mexico. Although it has not been reported in horses in the United States, it has been reported in donkeys.

The disease is transmitted by certain biting flies (including the tsetse) and ticks or by ingesting the feces of an infected cat. (Cats are the primary host of the protozoa.) The organisms form cysts in and under the skin and in blood vessels, mucous membranes of the upper respiratory tract, and other tissues. Other signs include fever, fluid buildup in the tissues, loss of appetite, intolerance to light, inflammation of the mucous membranes of the nose, chronic hardening and thickening of the skin, and hair loss. The skin becomes hard, thick, and wrinkled and develops cracks that allow bacterial infections to develop. Severely affected animals become emaciated. The signs in horses tend to be less severe than in other animals.

Affected animals should be isolated and treated for specific signs. Recovery is slow in severe cases, and affected animals remain carriers of the disease for life. Reducing the number of biting insects and ticks may reduce transmission.

Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Delores E. Hill, PhD; Barton W. Rohrbach, VMD, MPH, DACVPM; Charles J. Issel, DVM, PhD; Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Farouk M. Hamdy, DVM, MSc, PhD, MPA (Deceased); Kenneth R. Harkin, DVM, DACVIM; Johnny D. Hoskins, DVM, PhD; Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; John E. Madigan, DVM, MS; Dale A. Moore, MS, DVM, MPVM, PhD; J. Glenn Songer, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM; Brian J. McCluskey, DVM, MS, PhD, DACVPM; Bert E. Stromberg, PhD; Peter J. Timoney, MVB, MS, PhD, FRCVS